A porous poly(vinyl acetal) resin body is well known as a material having excellent water-absorbing and water-holding properties and showing, in a wet state, a desirable flexibility and an impact strength. With these properties, the porous poly(vinyl acetal) resin body is widely used as an optimum material for cleaning tools and wipers. Especially, because the body has a hydrophilic property and an excellent cleaning capability, and nevertheless does not damage objects scrubbed and, in addition, is excellent in strength, it has wide industrial applications such as cleaning of clean rooms, semiconductors, and electronic parts, and also plays an important role as a water absorbent material.
The porous poly(vinyl acetal) resin body herein is also called poly(vinyl acetal) sponge and will be abbreviated as “PVAt sponge” hereinafter.
A PVAt sponge is the most suitable material for cleaning clean rooms. To realize this advantage, the PVAt sponge should generally be used in a wet state, because the PVAt sponge hardens and loses its softness and elasticity, when dried. A water absorption rate of a porous PVAt body in a dry state is very different from that in a wet state, with the former being significantly slower than the latter. It takes much time to change the sponge from a dry state to a wet state, and therefore, it is practically preferred to keep it wet without drying it.
However, the porous PVAt type body tends to get moldy when kept wet for a long time. Mold causes problems in regard of hygiene, appearance and handling and, therefore, it is necessary to provide a porous PVAt type body with a mold-proofing property.
In storing the PVAt sponge, contamination of the porous body with organic substances, ionic substances, fine particles, and microorganisms shall be prevented. Especially in applications in the semiconductor industry, these contaminants should be reduced to the lowest possible limit. In a conventional method of storing the PVAt sponge, the sponge is impregnated with an agent having mold-proofing and pasteurizing properties, such as an aqueous solution of sodium dehydroacetate, 4-chloro-3, 5-dimethylphenol, benzalconium chloride, and benzimidazol.
However, when the PVAt sponge treated with a solution comprising those agents and stored is used as such for cleaning semiconductors or the like, these agents are eluted in cleaning water, i.e., ultra-pure water used for and contaminate the cleaning water and subjects to be cleaned. To remove these agents from the PVAt sponge treated with a solution comprising those agents and stored, the PVAt sponge is usually subjected to a long time of pre-cleaning before used.
In addition, when a PVAt sponge is treated with those agents and stored, the porous body is changed in qualities or degraded by the effects of some agents, which is also a problem.
Particularly, when a PVAt sponge is used as a cleaning material in the semiconductor industry or used as a water absorbent material in the electronics industry, for example, cleaning materials for precision cleaning of silicon wafers and photomasks, or for cleaning of printed circuit boards and lead frames or used as a water absorbent, foreign elements eluted from the PVAt sponge not only degrade qualities of products but also contaminate the production systems and environment itself, causing a huge damage. Thus, in applications in clean rooms or in an environment with a highly controlled cleanliness, the foreign elements originated from the porous body should be reduced extremely, but the above-mentioned storing method using agents could not meet this requirement.
To deal with these problems, the following methods may be thought: autoclave pasteurization under heat and pressure; pasteurization with an oxidizing gas such as ethylene oxide and ozone; storing a product in a sealed container and replacing a gas in the container with an inert gas; and pasteurization with UV light. For other materials than the PVAt sponge, an appropriate method was properly selected and applied among those methods. However, none of the methods is suitable to the PVAt sponge. For example, in the autoclave pasteurization, the material itself shrinks and loses its softness and elasticity, peculiar to the PVAt sponge. In the pasteurization with an oxidizing gas, it is difficult for the gas to penetrate deeply into the porous body and the porous body itself tends to deteriorate by oxidation. In the method where a product is stored in a sealed container and a gas in the container is replaced with an inert gas, it is not easy to remove oxygen dissolved in water adhered on the porous body and in the body itself and, consequently, mold growth cannot be prevented completely. The method utilizing UV light is not practical because UV light affects only the surface of the porous body.
As a result of intensive researches, the present inventor have found that, it is possible to prevent mold growth during prolonged storage of a PVAt sponge in a wet state and to avoid elution of substances or rising of dusts from the PVAt sponge in use after the storage to thereby reduce or eliminate a pre-cleaning process before use. The purpose of the present invention is to provide a PVAt sponge having a high degree of cleanliness, a method of preparing the same and a method of storing the same which allows strict preservation and easy handling of the cleanliness of the PVAt sponge having a high degree of cleanliness.